Experiments are underway to investigate the role of the phosphofructokinase-fructose diphosphatase substrate cycle in the rapid hormonal regulation of gluconeogenesis. These studies will involve the use of in vivo isotope techniques to investigate enzyme activities during different hormonal states. BIBLIOGRAPHIC REFERENCES: Singer, A. L., R. P. Sherwin, A. Dunn, and M. M. Appleman. Cyclic nucleotide phosphodiesterases in neoplastic and non-neoplastic human mammary tissues. Cancer Res. 36: 60-66 (1976). Dunn, A., K. Bever, M. Chenoweth, and J. Katz. The estimation of glucose recycling in vivo using H3 and C14 labeled glucose. Fed. Proc. 34: 465 (1975) (Abstract).